Naturally conducted or intrinsic ventricular depolarizations have been recognized as being preferable over ventricular pacing in general and pacing in the right ventricular apex in particular. In order to minimize or greatly reduce ventricular pacing, protocols have been developed that, in general, utilize an atrial based timing mode that promotes intrinsic conduction whenever possible. Illustrative protocols are described, for example, in U.S. Pat. No. 7,218,965 (Casavant), U.S. Pat. No. 6,772,005 (Casavant), and U.S. Pat. No. 7,248,924 (Casavant), all of which are incorporated herein by reference in their entireties.
In general, during a minimum ventricular pacing (MVP) protocol, atrial-only pacing is delivered unless intrinsic ventricular events are not sensed. When a ventricular event is not sensed between two consecutive atrial events, a ventricular backup pacing pulse may be delivered after the second atrial event to avoid another cardiac cycle of asystole. When a specified number of atrial-only pacing cycles occur without sensing a ventricular event, the pacing device switches to a dual chamber pacing mode and delivers atrial and ventricular pacing pulses coordinated at a programmed atrial-ventricular (AV) delay.
Accurate sensing of intrinsic cardiac activity is important in maintaining the appropriate response of the pacing device and properly switching pacing mode without undue delay when changes in atrial-ventricular conductivity occur. When an atrial event is sensed, an atrial refractory period (ARP) is set. The atrial tissue cannot be depolarized again until after a physiological refractory period has expired and any earlier signals sensed by the pacing device during the atrial refractory period are likely to be far-field signals or other non-atrial events. As such, any “events” sensed during the ARP may be noted by the pacing device but are not used in resetting pacing escape interval timers used to control the timing and delivery of pacing pulses. When a ventricular event is sensed, a post-ventricular atrial refractory period (PVARP) is set. Any “events” sensed using atrial sensing electrodes during the PVARP may be noted but are also not used for resetting any escape interval timers used for controlling pacing intervals. The PVARP is useful in preventing far-field R-waves from being sensed as P-waves.
Premature ventricular contractions (PVCs) or undersensing of an atrial P-wave may cause two consecutive ventricular sense events with no intervening atrial sense event. In these situations a PVARP is set following the second ventricular sense event, which may be an extended PVARP. The next atrial event may occur during the PVARP resulting in a pattern of atrial refractory sense (AR) events followed by ventricular sensed (VS) events, i.e., an AR-VS pattern. This AR-VS pattern can become sustained as each subsequent ventricular sense event is followed by a PVARP, during which the next intrinsic atrial event occurs. The AR-VS pattern can cause a delay in the first back-up ventricular pacing pulse when AV conduction block returns and can cause user confusion when the AR-VS sensing pattern is observed. Pacing devices typically record counts of atrial and ventricular paced and sensed events to determine and report paced events as a percentage of the events counted. This reporting provides the clinician with a measure of the amount of pacing that is occurring. AR-VS event intervals may not be counted in such device diagnostics (since AR events are commonly ignored) resulting in the percentage of pacing being over-reported. A need remains, therefore, for improvements in MVP methods that avoid the aforementioned consequences of sustained AR-VS sensing patterns.